Elect. Guru help

[bubblehide]

Originally Posted by GregAndrew

Am I wrong, or is there something more to these cells than just placing the right combination of them in series and parallel in order to get your required voltage and Ahs? All the cells seem to have a different discharge rate. At our usage, does that even matter?

Greg, your right. LION batteries produce heat as they discharge. The faster the discharge rate, the greater the heat produced (enough to melt right through a plastic kayak (recall the airline issues not that long ago). However, for the most part, placing small batteries in series/parallel hasn't up-ed the discharge rate to a point that would create a problem.

[GregAndrew]

Originally Posted by bubblehide

Greg, your right. LION batteries produce heat as they discharge. The faster the discharge rate, the greater the heat produced (enough to melt right through a plastic kayak (recall the airline issues not that long ago). However, for the most part, placing small batteries in series/parallel hasn't up-ed the discharge rate to a point that would create a problem.

Yeah, that is not where I was going with my question. Seems to me that any batteries that are charged or discharged too quickly beyond their ratings will have a tendency to generate heat. Although that is a risk that everyone should be aware of when making their wiring connections to avoid shorts.My concern was that some of the cells seem designed for rapid discharge and some for slower discharge. And I was wondering if that was going to effect performance (battery life or run time). Also, I am not looking at LION batteries, but Lithium Iron Phosphate batteries (LifePo).

[jruiz]

Originally Posted by GregAndrew

Yeah, that is not where I was going with my question. Seems to me that any batteries that are charged or discharged too quickly beyond their ratings will have a tendency to generate heat. Although that is a risk that everyone should be aware of when making their wiring connections to avoid shorts.My concern was that some of the cells seem designed for rapid discharge and some for slower discharge. And I was wondering if that was going to effect performance (battery life or run time). Also, I am not looking at LION batteries, but Lithium Iron Phosphate batteries (LifePo).

Short answer to your question, no. Long answer is that cell designers adjust the rate capability of a component cell by varying the thickness of the electrodes which directly affect your effective current density. Thicker electrode cells have lower current capability but higher energy density and thinner electrode cells the opposite.

Pack designers on the other hand adjust the current capability by adjusting the number of cells in parallel. Energy density in this instance stays constant and capacity increases.

Heat, fast charge/discharge, and the state you leave your battery at have the most impact on cycle life and irreversible capacity loss. Most battery manufactures ship batteries between 25-50 percent state of charge to maximize battery life and safety.

[bus kid]

Originally Posted by bubblehide

Greg, your right. LION batteries produce heat as they discharge. The faster the discharge rate, the greater the heat produced (enough to melt right through a plastic kayak (recall the airline issues not that long ago)

bad example, heat was not the issue for the airline, the issue was the gases emitted by the batteries not being able to be vented properly as the box containing them was too small. which caused a small less then 1 inch blue flame to be produced.